Combined intercom and door opener system



Nov. 8, 1966 G. WlTTlG COMBINED INTERCOM AND DOOR OPENER SYSTEM FiledFeb. 18, 1963 5 SheetS-Sheet l Nov. 8, 1966 G. WIT-nca COMBINED INTERCOMAND DOOR OPENER SYSTEM 5 Sheets-Sheet Filed Feb. 18, 1965 Vst ZA MPL/F/E/e Nov. 8, 1966 G. wlTTlG COMBINED INTERCOM AND DOOR OPENER SYSTEM 5Sheets-Sheet 3 Filed Feb. 18, 1963 Fig. 3

BELL? United States Patent Otiice 3,284,571 Patented Nov. 8, 19663,284,571 COMBINED INTERCOM AND DOOR OPENER SYSTEM Gerhart Wittig,Mannheim, Germany, assgnor to Handelslrontor Heider GmbH., Westphalia,Germany Filed Feb. 18, 1963, Ser. No. 259,040 Claims priority,application Germany, Feb. 24, 1962, H 45,001; Dec. 1, 1962, H 47,569 7Claims. (Cl. 179-2) The present invention concerns a combined intercomand door opener system of the general type conventionally `used inb-uildings, particularly in buildings containing one or more apartmentsand one nonmally closed door, such system comprising facilities forenabling a caller at the door t-o talk to a party in an apartment, andvice versa, and for enabling the party in the apartment to lopen thedoor by remote electrical control.

It is an object of the invention to provide for a system of the type setforth containing a comparatively small number of components so that thesyste-m 'can be installed at low cost for time and material.

lt is another object of t-he invention to provide for a system asmentioned above which requires between the door station and thecooperating station in .any one of the apartments only a two-lineconnection so that the system according to the invention can beinstalled in a particularly inexpensive and simple manner if in therespective building already a two`line connection exists between thearea of the door and the area of :the respective apartment orapartments.

With above objects in view the invention includes a combined intercomand door opener system with a door station and at least 4one remotelcooperating station, comprising, in combination, a door station havingtwo first terminals and including first elect-roacoustical transducermeans and electrical door opener means connected in parallel with each`other to said terminals; a central station having two second terminal-srespectively connected with said first terminals, and including twothird terminals. amplifier .means connected between said second andthird terminals lfor cooperating with said transducer means, a pluralityof relay means for controlling the operation of said amplifier means andthe operation of said door opener means, respectively; and supply meansfor applying electrical energy to said relay means, amplifier mea-ns anddoor opener means, respectively, at least one cooperating station havingtwo fourth terminals and including second electro-acoustical transducermeans and control means including switch means for controlling theoperation of said second transducer means and the ope-ration of saidrelay means, respectively, said second tnansd-ucer means and saidcontrol means being connected with said 'fourth terminals; and ltwo-lineconnection means arranged between said two third terminals and said twoIfounth terminals, respectively, for transmitting control criteria fromsaid switch means in said cooperating station to said relay means insaid central station and for transmitting signals between said first andsecond electroaco-ustical transducer means.

The novel features which are considered as characteristic for t-heinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation together with additional objects and advantages thereof, willbe best understood from the following description yof specificembodiments when read in connection with the accompanying drawings, inwhich:

FIG. -1 is a schematic circuit diagram illustrating one embodiment ofthe invention including a door station, a central station and one ofcooperating stations in an apartment;

FIG. 2 is a circuit diagram illustrating the main amplifier arrangementin the central station according to FIG. l;

FIG. 3 is a yschematic circuit diagram of a second embodiment of theinvention including a door station, a central station and a cooperatingstati-on in one of the apartments; and

FIG. 4 is a partial circuit diagram illustrating details of the mainamplifier arrangement in the central station of FIG. 3.

The system or installation according to FIG. 1 comprises a door stationTST, a central station ZST and at least one cooperating station WST inany one of the apartments of the respective building. It is evident thatany number of such cooperating stations may be connected in the mannershown with the two-line connection St which may be assumed to extend.through the entire building.

Each of the cooperating vapartment stations WST comprises anelectro-acoustical transducer RL which may be a conventional type ofaloud speaker which can also be used as a microphone, moreover achangeover switch I shown in its 4normal position, a singleat-hrowdouble-pole switch II the switch portions Ill and 112 -t'hereof beingheld preferably by spring means, not shown, in open position, and acapacitor C1 'arranged in the circuit of the transducer RL. The justlisted components are connected with each other and with the terminals aand b", respectively, as shown in FIG. 1.

The central station ZST containing an amplifier may be located 4anywherein the building, e.=g. in the basement, or in any event not too far`away from the door station. This central station ZST comprises anamplifier arrangement V including control means as indicated by IFIG. 2,a speech direction reversing relay RS (its Icontacts rs sh-own in FIG.2), 'a door opener relay RT with relay contacts rt, and including a.series resistor RV, and finally an amplifier starting relay RE, itscontacts re shown in FIG. 2. The last mentioned relay RE is providedwith conventional means for delaying its deenergization after thecurrent supply has been cut off. By way of example these delaying meansmay comprise a capacitor C2 connected in parallel with the relay coil REin combination with a series-connected rectifier or diode G.

Since the .amplifier V is only provided for amplifying speech `currentsor similiar signals transmitted between the electro-acousticaltransducers of the system, this amplifier V is blocked against directcurrent inpnt by capacitors C3 and C4 arranged between the amplifier andthe terminals a Iand b to which the above mentioned twoline connectionSt is connected. Similarly the other side of the amplifier V is blockedagainst direct current input by capacit-ors C5 and C6 arranged betweenthe amplifier V and the terminals c and d.

The central station ZST comprises further supply means for furnishingdirect current to the above mentioned relays and to the amplifier V, andfor furnishing alternating current to the door opener as describedfurther below.

The direct current supply may comprise conventional means like atransformer, rectifier, current smoothing device, etc., not shown andsymbolically indicated by a battery B. The supply of alternating currentis indicated at D.

The door station TST comprises an electro-acoustical transducer TL ofthe same type as the above described transducer RL, a conventionalelectrical or electro-mechanical door opener To and decoupling capacitorC7. The devices To and TL are connected in parallel with the terminals eand f, these terminals being connected with terminals c and d of thecentral station ZST by a twoline connection L as shown in FIG. 1.

FIG. 2 illustrates details of the amplifier arrangement V of FIG. l. Itcan be seen that energization of the relay RE will result in switchingon the conventional amplifier Vst by closing the relay contact re. Itcan be seen further that the input and output lines of the amplifier Vstare connected in the manner shown by FIG. 2 with the terminals a and b,i.e. with the two-line connection St, as Well as with the terminals cand d i.e. with the twoline connection L, via relay contacts rs whichare in the positions shown by FIG. 2 when the relay RS is not energized,but change to the respective second positions when this relay RS isenergized. Thus, by energization of the relay RS the input connection ofthe amplifier is exchanged against the output connection, and viceversa.

The operation of the arrangement according to FIGS. 1 and 2 is asfollows. After a caller at the door has alerted in conventional manner,e.=g. by ringing a bell, the party in the apartment where thecooperating station WST is installed, the alerted party would operatethe change-over switch I so as to move it from its normal position toits second position whereby the transducer RL is connected with theterminals a" and b" and thereby with the two-line connection St.Hereafter the double- -pole switch II is actuated so that its arms IIIand II2 are moved to closed position. Hereby communication between thestation WST and the door station TST is established. This is due to thefact that the switch arm -II2 in closed position shunts the capacitor C1whereby a direct current circuit is established as follows: from batteryB via rectifier G, -relay coil RE, terminal b, line b', terminal b",transducer RL, closed switch 112, switch I, terminal a, line a',terminal a and back to the battery B. Thus, the relay RE is energizedand closes its contact re (FIG. 2) so as to switch on the amplifier Vstwhereby the amplifier arrangement V is rendered operative.

Simultaneously with the energization of the relay RE also the delayingcapacitor C2 is charged and the relay RS is energized. Consequently, therespective relay contacts rs shown in FIG. 2 are moved from theillustrated position to their respective second position whereby theconnection coming from the cooperating station WST is connected with theinputs of the amplifier Vst while its outputs are connected with theterminals c and d and thereby with the transducer TL in the door openerstation TST. Thus, it is now possible to talk from the station WST tothe door station TST.

If now, while the change-over switch I is still in its second position,the direction of talk should be reversed so that speech can betransmitted from the transducer TL after amplification to the transducerRL, then the party at the station WST has only to release the doublepoleswitch II whereby the previously established circuit is immediatelyinterrupted and the relay RS is deenergized. As can be seen from FIG. 2the relay switches rs return now to their original normal position asillustrated so that now the input and output connections of theamplifier Vst are exchanged and speech can be transmitted from thetransducer TL to the transducer RL. The relay RE, however, has not beendeenergized when the circuit was interrupted by the return of the switchII to its normal position because, as mentioned above, the relay RE isprovided with delay means e.g. with the capacitor C2 in cooperation withthe rectifier G. However, it should be understood that the desired delaycan be achieved also by other conventional means eig. by means 0f aclockwork controlling a current supply. The diode G is provided in orderto prevent that the discharge of the capacitor C2 influences also therelay RS or RT, or both.

It can be seen that as long as the change-over switch I remains in itssecond position conversation between the stations can be carried out byalternatingly actuating the double-pole switch II as the change in thedirection of the signal transmission changes. However, the door openerrelay RT has not 4been energized up to now because this relay requires acurrent of greater strength than that of the current sufficient forenergizing the relays RE and RS. For this purpose the relay coil of therelay RT may have a corresponding higher resistance 0r a series-resistorRV may be combined with that coil.

It should be noted that every time the double-pole switch II isdepressed the relay RE is again energized and that every time the-double-pole switch is released the delay means mentioned above maintainfor a predetermined period of time the energization of the relay RE. Itis evident that the delay means can be chosen so that the predetermineddelay time is long enough to meet prevailing conditions.

If the conversation -between the stations is completed the change-'overswitch I is returned to its normal position as illustrated in FIG. 1whereby the transducer RL is disconnected. If it is now desired to openthe door the party at the station WST has only to actuate once more thedouble-pole switch II whereby the transducer RL and thereby itsresistance is shunted by the closing of the switch arm III. This has theeffect that now a current of ygreater strength is permitted to owthrough the thus established circuit and that thereby now the -relay RTis energized in spite of its having a greater resistance, or in spite ofthe presence of the resistor RV, whereby the frelay contact rt is closedand alternating current from the supply D is applied to the door openerTo. It is immaterial that at the same time also the relays RE and RS areenergized because even if now the amplifier arrangement V is thusrendered operative this has no effect because the transducer RL has beendisconnected. Incidentally, after the end of the predetermined delaytime the relay RE is deenergized anyway so that thereafter the amplifieris not operating.

The above described embodiment may be modified in various ways withoutdeparting from the gist of the invention. For instance, the controlcriteria transmitted from Ithe station WST for establishingcommunication and for opening the door may not be :constituted byvarying the strength of a direct current flowing through the system. Ina similar manner alternating currents lhaving different characteristicsmay be used e.g. currents differing in amplitude as well as currentsdiffering in phase.

The embodiment illustrated by FIGS. 3 and 4 differs from the abovedescribed embodiment in various ways. The station WST' is quite similarto the station WST of FIG. 1 except that a 'resistor R1 is connectedbetween the switch arm II2 and the capacitor C1, and a second resistorR2 is connected in parallel with the transducer RL.

In the central station ZST an auxiliary amplifier VRE is provided foramplifying even a weak current sufficiently for energizing theassociated rel-ay RE. Thus, the relay RE in combination with theauxiliary amplifier will respond to a current of lesser strength thanthat to which the relay RS will respond while, as in the `abovedescribed example, the relay RT will require a current of greaterstrength than that which suffices to energize the relay RS. Thecapacitors C3, C5 and C6 are omitted. The amplifier arrangement V isconstructed in detail fas shown by FIG. 4, exactly like the amplifier)arrangement V according to FIG. 2. Also the door .station TST isidentical with the same station as illustrated in FIG. 1.

The arrangement according to` FIGS. 3 :and 4 operates as follows. Afterthe party at the station WST' has been alerted e.g. by the ringing olf abell, the party may move the change-over switch I from the normalposition illustrated by FIG. 3 to its second position whereby thetransducer RL is connected with the terminals a and bi" and therebythrough lines a and b with t-he terminals a and b. However, at the sa'metime the resistor R2 is connected across the lines a' and b' so that aow of current is caused as follows: from the positive terminal of thedirect current supply via the relay RE, relay contacts ft2 (describedfurther below), amplifier VRE (which in this example is continuouslyoper-ative), terminal b, line b", terminal b, resistor R2, change-overswitch I, terminal a, line a', terminal a `and back to the negativeterminal of the supply.

In this now established circuit the resistance of the resistor R2 is sohigh that t-he current flow in this circuit is not suflcient to energizethe relays RS and RT, w-hile the relay RE is energized by thecomparatively weak current owing through the circuit because of thepresence of the auxiliary amplifier VRE. The energization of relay RE`closes the contact re (FIG. 4) whereby the amplifier Vst is switchedon. Since the relay RS requiring a stronger c-urrent has not beenenergized the amplifier is now connected via the relay contacts rs1-r5.1in the position illustrated by FIG. 4 between the transducer TL and thetransducer RL so that now a ca'ller at the door station TST can talk tothe party at the station WST.

It can be seen that the advantage of this arrangement resides in thefact that after :actuation of the changeover switch I alone the relay REis energized so that the conversation from the door station to thestation in the apartment can be carried out for any desired length oftime as long as the switch I remains in its second position. Thus, therelay RE does not have to be equipped with delay means.

If now the party at the station WST wishes to talk to the party at thedoor station TST the double-pole switch II has to be actuated while thechange-over switch I still remains in its second position. Hereby theresistor R1 is connected in parallel with the resistor R2 due to theclosing of the switch portion lI2. Now the combined resistance is so lowthat the relay RS is energized by the current flowing as follows: fromthe positive terminal of the direct current sup-ply B through the relaycoil RS, terminal b, line b', terminal b, lresistors R1 and R2, switchI, terminal a, line a', terminal a, and back to the negative terminal ofthe source B. However, it has to be understood that the thus establishedcirc-uit still is not able to carry enough current for energizin-g therelay RT. The energization of the relay RS causes the relay contactrs1-m4 to move to their respective second position whereby the input andoutput connections of the amplifier Vst are exchanged against each otherso that signals can be transmitted from the transducer RL through theIamplifier to the transducer TL. If thereafter the direction of thetransmission of signals should be reversed again the party at thestation WST' has -only to release the double-pole switch II so that theswitch portion H2 returns to its normal position which causes the relayRS t-o be deenergized.

When the conversation between the stations is terminated the change-overswitch I is returned to its normal position illustrated in FIG. 3whereby the transducer RL is switched off; at the same time the currentow through the resistor R2 is interrupted so that Ithe relay RE isdeenergized and the amplifier Vst is also switched oif by the opening ofthe relay contact re.

If now, with the change-over switch I in normal position, thedouble-pole switch II is :again actuated a direct connection between thelines a and b is established through the switch portion H1.Consequently, any resistance existing in the station WST' is removedfrom the circuit and a rather strong current can now ow from thepositive terminal of the source B through the relay RT, resistor RV,terminal b, line b', terminal b, switch Ill, switch I, terminal a",Iline a', terminal a and back to the negative terminal of the source B.This current flow is strong enough to energize the relay RT.Consequently the relay contact rtl is changed from the normal positionillustrated in FIG. 3 to its second position whereby the circuit betweenthe alternating current source D and the door opener To is completed viathe two-line connection L. At the same time the change of position ofthe relay contact rtl separates the amplifier arrangement V from thealternating current supply whereby possible damage is avoided. Inaddition, the second relay contact ft2 of the relay RT interrupts thecirc-uit for the relay RE. It so happens that the relay RS is energizedsimultaneously with the relay RT 'but this is without any consequencebecause the amplifier arrangement is switched off.

In certain cases it may be desirable to utilize the alternating currentsupply D in the central station ZST also for operating a bell or buzzerconnection between the door station and the station in the particularapartment or apartments. This can be done with the additional advantagethat one of the lines of the two-line connection Sl is also utilized inthe bell or buzzer system. This possibility is illustrated in FIG. 3.For this pu-rpose one line of the alternating current supply D isconnected permanently by a connection g via terminal a with the line a.A bell or buzzer K is connected at one side with the line a while it isconnected at the other side by a line i1, a bell button P1 and line hvia terminal f and d with the other line of the alternating currentsupply D. By pressing the bell button P1 the bell or buzzer K issupplied with alternating current as can be seen readily. If there aremore apartments in the system additional lines i2 and bell buttons P2may be added as required. Thus, one line for a bell or buzzer system andan extra transformer is saved in this modified installation.

It should not be overlooked that the arrangement according to theinvention entails the further advantage that, provided that theadditional bell or buzzer installation is not present as shown, it isimmaterial whether the line a is connected with terminal a and line b isconnected with terminal b, or whether line a is connected with terminalb and line b is connected with terminal a". Similarly it is immaterialwhether the terminal c is connected With terminal e, terminal d withterminal f, or whether terminal c is connected with terminal f andte-rminal d with terminal e. This feature greatly facilitates theinstallation of the various stations described above in a building,particularly if already a two-line connection exists which only has tobe tapped for utilizing it for the connections between the stations.

It will be understood that each of the elements described above or twoor more together, may also find a useful application in other types of acombined intercom and door opener system differing from the typesdescribed above.

While the invention has been illustrated and described as embodied in acombined intercom and door opener system with a door station and atleast one remote cooperating station, it is not intended to be limitedto the details shown, since various modifications and structural changesmay be made without departing in any way from the spirit ofthe presentinvention.

Without further analysis the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed and desired to be secured by Letters Patent is:

l. A combined intercom and door opener system with a door station and atleast one remote cooperating station, comp-rising, in combination, adoor station having two first terminals and including firstelectro-acoustical transducer means and electrical door opener meansconnected in parallel with each other to said terminals; a centralstation having two second terminals respectively connected with saidfirst terminals, and including two third terminals, amplifier meansconnected between said Second and third terminals for cooperating withsaid transducer means, a plurality of relay means for controlling7 theoperation of said amplifier means and the operation of said door openermeans, respectively; and supply -means for applying electrical energy tosaid relay means, amplifier means and door opener means, respectively;at least one cooperating station having two fourth terminals andincluding second electro-acoustical transducer means and control meansincluding switch means for controlling the operation of said secondtransducer means and the operation of said relay means, respectively,said second transducer means and said control means being connected withsaid fourth terminals; and two-line connection means arranged betweensaid two third terminals and said two fourth terminals, respectively,for transmitting control criteria from said switch means in saidcooperating station to said relay means in said central station and fortransmitting signals between said first and second electroacousticaltransducer means.

2. A combined intercom and door opener system according to claim 1,wherein said plurality of relay means includes first relay means forconnecting, when energized, said amplifier means with said supply means,second relay means for interchanging, when energized, the connectionsbetween said second and third terminals, respectively, and the input andoutput, respectively, of said amplifier means, and third relay means forconnecting, when energized, said door opener means with said supplymeans, said third relay means requiring for its energization a currenthaving a characterstic different from that of currents required by saidfirst and second -relay means, and wherein said control means isconstructed so as to determine, depending upon actuation of said switchmeans, said characteristics of said currents energizing said variousrelay means, respectively.

3. A combined intercom and door opener system according to claim 1,wherein said plurality of relay means includes first relay means forconnecting, when energized, said amplifier means with said supply means,second relay means for interchanging, when energized, the connectionsbetween said second and third terminals, respectively, and the input andoutput, respectively, of said a-mplifier means, and third relay meansfor connecting, when energized, said door opener means with said supplymeans, said third relay means requiring for its energization a currentof greater strength than that of currents required by said first andsecond relay means, and wherein said control means is constructed so asto determine, depending upon actuation of said switch means, thestrength of said currents energizing said various relay means,respectively.

4. A combined intercom and door opener system according to claim 1,wherein said plurality of relay means includes first relay means forconnecting, when energized, said amplifier means with said supply means,second relay -means for interchanging, when energized, the connectionsbetween said second and third terminals, respectively, and the input andoutput, respectively, of said amplifier means, and third relay means forconnecting, when energized, said door opener means with said supplymeans, said first relay means including a relay and currentamplification means for raising the strength of current applied theretoto that required for energizing said relay, said third relay meansrequiring for its energization a current of greater strength than thatof the current required by said second relay means, and said first relaymeans requiring a current of lesser strength than that required by saidsecond relay means, and wherein said control means include resistormeans, a change-over switch and a singlethrow double-pole switchconnected with each other and with said second transducer means in sucha manner that when said change-over switch is in its normal position andsaid double-pole switch is closed a current of suflicient strength iscaused to flow through the thus established circuit to energize saidthird relay means, and that when said change-over switch is moved to itssecond position while said double-pole switch is in its normal openposition only that amount of current is caused to flow through the thusestablished circuit which suffices only to energize said first relaymeans and said first and second transducer means are operativelyconnected, and that when said change-over switch is in its secondposition and additionally said double-pole switch is moved to closedposition a current is caused to fiow through the thus establishedcircuit which suffices to energize said second relay means and saidfirst and second transducer means are operatively connected.

5. A combined intercom and door opener system according to claim 1,wherein said plurality of relay means includes first relay means forconnecting, when energized, said amplifier means with said supply means,second relay means for interchanging, when energized, the connectionsbetween said second and third terminals, respectively, and the input andoutput, respectively, of said amplifier means, and third relay means forconnecting, when energized, said door opener means with said suppymeans, said third relay means requiring for its energization a currenthaving a characteristic different from that of currents required by saidfirst and second relay means, said first relay means including means`for delaying through a predetermined time period the deenergizationthereof after the energy supply thereto has been cut off, and whereinsaid control means is constructed so as to determine, depending uponactuation of said switch means, said characteristics of said currentsenergizing said various relay means, respectively.

6. A combined intercom and door opener system according to claim 1,wherein said plurality of relay -means includes first relay means forconnecting, when energized, said amplifier means with said supply means,second relay means for interchanging, when energized, the connectionsbetween said second and third terminals, respectively, and the input andoutput, respectively, of said amplifier means, and third relay means forconnecting, when energized, said door opener means with said supplymeans, said third relay means requiring for its energization a currentof greater strength than that of currents required by said first andsecond relay means, said first relay means including mean for delayingthrough a predetermined time period the deenergization thereof after theenergy supply thereto has been cut off, and wherein said control meansis constructed so as to determine, depending upon actuation of saidswitch means the strength of said currents energizing said various relaymeans, respectively.

7. A combined intercom and door opener system according to claim 6,wherein said plurality of relay means includes first -relay means forconnecting, when energized, said `amplifier means with said supplymeans, second relay means for interchanging, when energized, theconnections between said second and third terminals, respectively, andthe input and output, respectively, of said amplifier means, and thirdrelay means for connecting, when energized, said door opener means withsaid supply means, said third relay -means requiring for itsenergization a current of greater strength than that of currentsrequired by said first and second relay means, said first relay meansincluding means for delaying through a predetermined time period thedeenergization thereof after the energy supply thereto has been cut ofi,and wherein said control means include a change-over switch and asingle-throw double-pole switch connected with each other and with saidsecond transducer means in such a manner that when said change-overswitch is in its normal position and said double-pole switch is moved toclosed position 9 a current of suficient strength is caused to ilowthrough the thus established circuit to energize said third relay means,and that when said change-over switch is moved to its second positionand additionally said double-pole switch is moved to closed positiononly that amount of current is caused to flow through the thusestablished circuit which suces to energize `only said first and secondrelay means and said rst and second transducer means are operativelyconnected, and that when thereafter said change-over switch is in itssecond position and 10 said double-pole switch is returned to its normalopen position said second relay means is deenergized While ReferencesCited by the Examiner UNITED STATES PATENTS 1,857,107 5/1932 Cadieux179-2 2,372,435 3/1945 LaForest 317--137 X 2,387,443 10/ 1945 Hasenberg179-1.4

DAVID G. REDINBAUGH, Primary Examiner.

J. T. STRATMAN, Assistant Examiner.

1. A COMBINED INTERCOM AND DOOR OPENER SYSTEM WITH A DOOR STATION AND ATLEAST ONE REMOTE COOPERATING STATION, COMPRISING, IN COMBINATION, A DOORSTATION HAVING TWO FIRST TERMINALS AND INCLUDING FIRSTELECTRO-ACOUSTICAL TRANSDUCER MEANS AND ELECTRICAL DOOR OPENER MEANSCONNECTED IN PARALLEL WITH EACH OTHER TO SAID TERMINALS; A CENTRALSTATION HAVING TWO SECOND TERMINALS RESPECTIVELY CONNECTED WITH SAIDFIRST TERMINALS, AND INCLUDING TWO THIRD TERMINALS, AMPLIFIER MEANSCONNECTED BETWEEN SAID SECOND AND THIRD TERMINALS FOR COOPERATING WITHSAID TRANSDUCER MEANS, A PLURALITY OF RELAY MEANS FOR CONTROLLING THEOPERATION OF SAID AMPLIFIER MEANS AND THE OPERATION OF SAID DOOR OPENERMEANS, RESPECTIVELY; AND SUPPLY MEANS FOR APPLYING ELECTRICAL ENERGY TOSAID RELAY MEANS, AMPLIFIER MEANS AND DOOR OPENER MEANS, RESPECTIVELY;AT LEAST ONE COOPERATING STATION HAVING TWO FOURTH TERMINALS ANDINCLUDING SECOND ELECTRO-ACOUSTICAL TRANSDUCER MEANS AND CONTROL MEANSINCLUDING SWITCH MEANS FOR CONTROLLING THE OPERATION OF SAID SECONDTRANSDUCER MEANS AND THE OPERATION OF SAID RELAY MEANS, RESPECTIVELY,SAID SECOND TRANSDUCER MEANS AND SAID CONTROL MEANS BEING CONNECTED WITHSAID FOURTH TERMINALS; AND TWO-LINE CONNECTION MEANS ARRANGED BETWEENSAID TWO THIRD TERMINALS AND SAID TWO FOURTH TERMINALS, RESPECTIVELY,FOR TRANSMITTING CONTROL CRITERIA FROM SAID SWITCH MEANS IN SAIDCOOPERATING STATION TO SAID RELAY MEANS IN SAID CENTRAL STATION AND FORTRANSMITTING SIGNALS BETWEEN SAID FIRST AND SECOND ELECTROACOUSTICALTRANSDUCER MEANS.